Building a Digital Clock Kit (HPR Show 2329)

Dave Morriss

Table of Contents

Introduction

In April 2017 my son and I decided to each build a digital clock. I had been interested in the idea since seeing Big Clive build one on YouTube, and I think my son had been similarly motivated.

He found one, which I have linked to below. It’s smaller than the one shown by Big Clive, comes from ShenZhen China, and currently costs $5.35 (about £4.18) postage free. It takes a long time to arrive, so patience is needed!

There are many digital clock kits on eBay, and lots of YouTube videos showing how to build them. I think it’s a great project for someone wanting some soldering practice which is a little more demanding than a beginner project.

One type to avoid, I think, is the surface mount type. The one I have uses a through-hole PCB, but I have seen some that provide SMD (surface-mounted device1) components. That type of soldering is beyond me at the moment (though my son has been teaching himself to do it).

Unpacking

I have included a number of images with the episode. What you see here are thumbnails. Click on them to view the full image.

The kit came in a standard bubble-wrap package, with some components bagged or shrink-wrapped together.

Picture: The components after un-boxing

Contents were a PCB, a 4-digit display, a perspex box, two chips and associated sockets, various components and a USB power lead. Contrary to what is stated on the eBay site, there is a battery included in the kit, a CR1220 Lithium Cell.

Picture: The components unwrapped

The PCB looked nice, so I photographed front and rear.

Pictures: The PCB component side and reverse side

Building

So, on to building the kit. There are some hints about the sequence of components. I started with the resistors.

In the image I show off the PCB holder I got for Christmas!

Picture: Starting to add components

The PCB holder can’t cope as the board becomes populated since there’s nothing to hold on to.

Getting the LDR (light dependent resistor) and the thermistor (temperature sensor) positioned so they protrude though the case needs care. In case you wondered I washed before removing the seal. ☺

Pictures: Most components added, front and back view of the PCB

I test assembled the case and wrote on each piece where it was to go. It was not too simple to get it right I found.

Picture: Putting the case together to see how it fits

The fitting of the display needed care since the pins were a bit splayed or bent and needed straightening. Orientation is important of course, as is clearance of components on the board (like the crystal, which could short out pins).

Pictures: Test fitting the display, showing clearance for the display pins. Then showing the display soldered on and pins cropped short

The clock was a tight fit in the case, which is why it was very important to ensure that everything was properly aligned on the PCB and the clearance between PCB and display chip were as small as possible.

Even so, the speaker did not match with the hole in the case. It was made to fit eventually by careful positioning (not brute force, though I was tempted!)

Pictures The clock installed in the case, but slightly misaligned. Various views

Everything was assembled and power applied, and it worked!

Taking photos of it in action is difficult.

Picture: It lives!

Instructions

In short, these are pretty bad! Mine consisted of a single sheet, badly photocopied and difficult to read in places.

There is a diagram of the PCB, which is helpful for component placement, as is the list of components with numbers like r1 and r2 matching the picture.

The written instructions are pretty bad though. The translation seems very poor using ‘welding’ for ‘soldering’ and many sentences are close to meaningless. Things like:

The pins with diagonal cutting pliers cut short (this step is important) as far as possible, avoid to resist digital tube affect beautiful.

Welding digital tube, digital tube must pay attention to the final, or placed on the back of the device can’t welding.

I think this means to ensure all component wires are trimmed as short as possible to avoid touching the display – which fits on the reverse of the PCB. This makes sense because there is a very tiny clearance for the PCB and display or they don’t fit in the case.

Picture: The instructions aren’t very good

Setting the clock

This was a bit of a challenge. I think that the process is:

Press the set (top) button once to change the time (hours). The change is performed by repeatedly pressing the add button.

Press set again to change the minutes of the time, using add as before.

Press set again to adjust the hours of the alarm, using add.

Press set again to adjust the minutes of the alarm, using add.

Press set again to enable/disable the alarm. Its state is shown by the light in the bottom right corner of the display (light off = alarm off)

Press set again to enter the mode controlling an hourly beep. Here add changes the left side of the display which defines the hour at which the beep is turned on (e.g. 08 = 8 a.m.)

Press set again and the right side of the display flashes, then use add to adjust. This defines the end time for hourly beeps (e.g. 20 = 20:00, 8 p.m.)

Press set again then use add to enable/disable this mode. The rightmost light on bottom of the display shows the mode is enabled when on.

Press set one more time and the clock is back to normal (normal walking as the instructions say).

The thing I try not to forget is that the set key needs 9 presses to cycle through all settings and back to normal.

Impressions

The clock is fine, and not bad for the price. On the other hand you get what you pay for!

The timekeeping is OK, though I have seen a bit of drift in the few weeks I have had it. The battery backup is good to have (though the battery used, a CR1220, is not quite as easy to find as most, according to my researches).

The clock shows the temperature every 30 seconds then returns to the time display. The temperature sensor is not accurate at all. I have my clock on top of a powered USB hub under one of my monitors. It may be warmer there than elsewhere, but 29°C seems high on a coolish day where other thermometers are reading 24°C in the house. Some means of calibration would be nice.

The light sensor (LDR) turns the brightness down when the ambient light level is lower, which is good since the display is very bright.